In radio transceivers, and particularly in direct-conversion receivers, amplifiers having low noise figures and capable of handling high-level signals are needed in front-ends of an in-phase (I) and a quadrature (Q) signal paths. FIG. 1 illustrates a receiver structure converting a received radio signal directly to the baseband. The receiver comprises a first amplifier 2 before mixers 4 and 5. The first amplifier 2 is typically a low-noise amplifier. Bandpass filters 1 and 3 have been provided before and after the amplifier 2 to remove undesired frequency components. Mixers 4 and 5 mix in-phase (I) and quadrature (Q) components of the received radio signal to the baseband with local oscillator signals LO_0, LO_90, LO_180, and LO_270. The number refers to the phase shift of the respective local oscillator signal. After the downmixing, baseband amplifiers 6 and 7 amplify the downmixed I and Q components, respectively, and low-pass filters 8 and 9 remove harmonic signal components resulting in the downmixing. Amplifiers 10 and 11 further amplify the low-pass filtered signals before analog-to-digital (A/D) conversion in an A/D-converter 12.
In case the baseband amplifiers 6 and 7 are active amplifiers, noise figures of the baseband amplifiers 6 and 7 are typically relatively poor due to flicker noise (known also as 1/f noise), among others. Nowadays, supply voltages applied to the active amplifiers 6 and 7 are quite low, which degrades their ability to handle input signals having large amplitudes. As a consequence, the amplifiers 6 and 7 may distort input signals severely causing difficulties in further processing of the input signals. Low-pass filters may be arranged to have low impedance levels in order to minimize noise levels. This may result in high capacitance values in the low-pass filter components and, as a consequence, increase the size of an actual implementation in an integrated circuit.